1. Field of the Invention
This invention is concerned with apparatus for measuring the porosity and permeability of reservoir rock cores under simulated in-situ conditions. More specifically, the invention relates to equipment for automatic sequential testing of a plurality of rock core samples. Such equipment might be found in class 73/38.
2. Discussion of the Related Art
As is well known, hydrocarbon reserves are found in porous subsurface rock formations. Usually such formations are of marine sedimentary origin such as vugular limestone or dolomite and porous sandstone. Of interest in determining reservoir characteristics are porosity and permeability. Porosity is measured as the percentage of void space in the rock, available to contain fluids, relative to a unit of gross rock volume. Permeability is a measure of the ease with which fluids can migrate between the voids in the formation. Permeability is measured in millidarcys.
For purposes of this disclosure, porosity and permeability are measured using cylindrical plugs cut from the reservoir rock. Typically, the plugs are about one or one and one-half inches in diameter. A core sample is inserted into a core holder where it is subjected to axial and radial stress to simulate the stresses existing at the original formation burial depth.
Typically, the core holder is a cylindrical pressure vessel. A flexible sleeve or cuff is mounted inside the core holder to encase the core sample. After core insertion, the annular space between the inner wall of the core holder and the sleeve is pressurized to apply radial stress to the core sample. The applied pressure is comparable to the static stress existing at the formation depth which may range from 500 to 10,000 psi. The core holder has a porous fixed upper end plug against which the upper end of the core sample is pressed. A lower end plug, usually attached to a hydraulic piston or plunger, contacts the lower face of the core. The piston applies an axial stress to the core sample, commensurate with the applied radial stress. The upper and lower end plugs as well as the end faces of the core sample must be accurately perpendicular to the longitudinal axis of the core holder so that the core sample is evenly stressed.
The upper end plug usually includes an inlet valve through which an inert gas may be injected in the pressure range of 50-500 psig, in inverse proportion to the permeability. The face of the lower piston or plunger has a vent that can be releasably sealed by a poppet valve that is mounted inside the piston. To measure porosity, the vent in the lower plunger is sealed and a known volume of gas is injected into the core sample. To determine the permeability, the vent is opened so that the gas flow rate through the core sample can be measured.
This invention is related to U.S. Pat. Nos. 4,561,289, 4,643,019 and 4,649,737, issued respectively on 12/31/85, 02/17/87 and 03/17/87, all of which are incorporated herein by reference as showings of apparatus to which my present teachings may be applied as well as to provide a disclosure of prior art of which I am presently aware.
The '737 patent teaches an apparatus for automatically testing a plurality of core samples on a mass production basis. Essentially, that apparatus consists of a rotatable carousel having a plurality of storage carriers mounted thereon. In use, the carousel is rotated so that a selected core sample, in its storage carrier, is aligned with a core holder that is mounted above the carousel. A transfer piston then pushes the core sample from the storage carrier, up into the core holder for conducting the tests as previously described. Following the test procedure, the core sample is lowered from the core holder back into the carrier and another core sample is selected for test.
The '289 and '019 patents disclose a core-contacting disk that is press-fitted into the upper end of the transfer piston. The disk is either perforated or is made from a porous metal. Gas distribution channels are formed on the bottom of the perforated disk ('289) or on the top face of the transfer piston ('019).
I have encountered problems with the devices as taught by those patents.
Particularly with sandstones, the sand grains of many cores are poorly cemented. During the tests, sand grains, clay particles and dust break away, to fall through the vent in the transfer piston. The sand grains contaminate the O-rings of the poppet valve, possibly abrading the O-rings, creating a poor seal, that results in inaccurate porosity tests. If the porous metal disk as disclosed in the '019 patent is substituted for the perforated end plug as disclosed in the '289 and '737 patents, although gross contaminants are eliminated from the poppet valve, dust particles from friable core samples clog the micro-pores, preventing accurate permeability measurements. Frequent disassembly and cleaning of the apparatus seriously impeded core-testing production, which ideally approaches more than 350 such tests each day. Fine-mesh screens, mounted between the lower end cap and the end plug, have been tried but the screens either became clogged or were punctured by the particulate matter. The screens proved to be ineffective.
The disks disclosed by the aforementioned patents are press-fitted into the end of the transfer piston. The disks tended to become dislodged and to become skewed at an improper angle with respect to the longitudinal axis of the core holder. The effect damaged the end faces of the cores under test, again resulting in inaccuracies in test results.